Conventional air conditioning systems provide heating and cooling to the interiors of buildings and other contained spaces via interior utilization side heat exchangers. During normal system operation, refrigerant flows through one or more utilization side heat exchangers before flowing through an exterior heat source side heat exchanger. After exiting the heat source side exchanger, the refrigerant enters a compressor, where its pressure and temperature are rapidly increased. The refrigerant then exits the compressor in liquid phase, as is known in the art.
However, the temperature of the refrigerant as it is discharged from the compressor must be below a predetermined maximum allowable temperature associated with the compressor. Specifically, if the temperature of refrigerant exiting the compressor exceeds the predetermined maximum allowable temperature, the compressor will likely fail. Conventionally, it is difficult to downwardly adjust the temperature of the refrigerant entering the heat source side heat exchanger prior to the refrigerant entering the compressor. Therefore, the refrigerant entering the compressor may result in a discharge temperature of the compressor that is above the maximum allowable temperature.
Japanese Patent Application Publication No. 2009-222357 describes an air conditioning system that is equipped with a refrigerant circuit including a compressor, condenser, an expansion mechanism, and first and second evaporators, respectively. A zeotropic refrigerant mixture circulates through the refrigerant circuit.
The refrigerant circuit also includes a pressure control device located between the first and second evaporators for reducing pressure of the refrigerant one or more times during the evaporation process as the refrigerant flows between the first and second evaporators. The decrease in pressure is ultimately helpful in decreasing the suction pressure of the refrigerant entering the compressor.
However, the refrigerant circuit does not decrease suction temperature of the refrigerant as it flows from the second evaporator to the compressor. Thus, the suction temperature of the refrigerant flowing into the compressor from the second evaporator may be above a tolerance, or in other words a predetermined maximum allowable temperature, of the compressor as the refrigerant flows from the compressor.
In addition, in the system above frost forms on the heat source side heat exchanger during system operation. When the system is operated in a defrost mode, the maximum opening degree of the pressure control device is small. As a result, very little refrigerant passes through the pressure control device to circulate through the refrigerant circuit, resulting in a shortage in system defrost capacity. If refrigerant is forced through the pressure control valve during the defrost mode, damage to the pressure control valve can occur.
There is therefore a need for a refrigerant circuit that can reduce the temperature of refrigerant flowing into a compressor from a heat exchanger to a level where the temperature of the refrigerant flowing from the compressor is within a fault tolerance of the compressor. There is also a need for a refrigerant circuit that can provide adequate condenser defrost capacity even when a pressure control device is present in the circuit.